GB2120840A - Contrast improvement in vacuum image display devices - Google Patents

Abstract

Loss of contrast due to back- scattered electrons from a fluorescent screen of a vacuum image display device can be reduced by applying a thin metallic reflecting film 34 to the back of the fluorescent screen and applying a layer 36 of an element of low atomic number and low back-scattering coefficient, for example carbon, to the reflecting layer. The thickness of the reflecting film is sufficient to obtain the desired reflection and to provide an impervious layer to prevent the element of low atomic number from passing through the metallic film. By applying a desired thickness of the element then an acceptable compromise between the back- scattered contribution and the primary light output can be reached for any given operating voltage. For a reflecting film of aluminium of 19 mu g/cm<2> thickness an empirical relationship to determine the operating conditions to give the required contrast performance is VC = 0.38VA + K where VC is the cut-off voltage on the screen, VA is the electron acceleration potential and K is a constant depending on the contrast C, i.e. the ratio between light output due to back-scattered electrons to light output due to primary electrons. <IMAGE>

Description

SPECIFICATION Improving contrast in vacuum image display devices The present invention relates to improving the contrast in vacuum image display devices which contain a fluorescent screen and require an accelerating voltage in the vicinity of the screen, for example some cathode ray tubes and storage tubes.

In vacuum display tubes in which electrons are accelerated through a large potential difference immediately before impact with the fluorescent screen, a particularly troublesome loss of contrast can occur due to electrons which are back-scattered in the collision process. These electrons, which can be as many as one-fifth of the primary electrons, are returned to the screen at some point remote from the initial point of contact with sufficient energy to generate a further contribution to the total light emission. This additional light is usually noticed as a halo surrounding a highlight, but in a complex scene of more uniform light distribution it causes an overall loss of contrast.

Many proposals for avoiding this deficiency by coating the screen with a layer of an element of low atomic number which will have a low back-scattering coefficient, have met with varying degrees of success. Carbon, atomic number 6, has been used most commonly as the screen backing as it is also a good electrical conductor and charging effects are avoided. Carbon, however, is not usually a good light reflector so that if it is used alone, the overall light output from the screen is significantly reduced.

It has also been proposed to have a thin film of aluminium to reflect light from the fluorescent screen forward to improve the overall light output. However the aluminium film generally is too thin to stop back-scattered electrons from the screen from re-entering the screen and producing a light output.

According to one aspect of the present invention there is provided a method of improving the contrast of an image displayed on a vacuum image display device containing a fluorescent screen and requiring an accelerating voltage in the vicinity of the screen, comprising applying a metallic reflecting layer to the fluorescent screen to reflect light produced by the screen and applying a layer of an element of low atomic number and low back-scattering coefficient on the metallic reflecting layer to reduce back-scatter of electrons from the screen, wherein the thickness of the metallic layer is selected to be sufficient to obtain the desired reflection and provide an impervious layer to prevent said element from going through the metallic layer into the screen and the thickness of the layer of said element is selected to give the desired contrast.

According to another aspect of the present invention there is provided a vacuum image display device comprising an evacuated envelope having a faceplate, a fluorescent screen adjacent to, or spaced from, the faceplate, a metallic layer on the fluorescent screen to reflect light produced by the screen towards the faceplate, and a layer of an element of low atomic number and low back-scattering coefficient on the metallic reflecting layer to reduce back-scatter of electrons from the screen, wherein the thickness of the metallic layer is such as to obtain the desired reflection and to form an impervious layer to prevent said element from going through the metallic layer into the screen and the thickness of the layer formed by the element of low atomic number is determined to give the required contrast performance in accordance with the following relationship:: Vc = 0.38VA + K where Vc is the cut-off voltage of the screen, VA the electron acceleration potential and K is a constant depending on the contrast C which is defined as the ratio between light output due to back-scattered electrons to light output due to primary electrons.

In an image display device made in accordance with the present invention an acceptable compromise between the back-scattered contribution and the primary light output is reached for any given operating voltage. In consequence it is possible to reduce the loss in contrast due to back-scattering to less than that due to other sources such as optical effects in the faceplate glass.

The element of low atomic number may comprise carbon.

The present invention will now be described, by way of example, with reference to the accompanying drawings, wherein: Figure 1 is a diagrammatic view of a display tube including a large area channel plate electron multiplier, Figure 2 is an enlarged detail of a display tube showing the final dynode and output electrode of the electron multiplier and, on a flat faceplate, a phosphor screen and layers of aluminium and carbon, Figure 3 is a graph of screen voltage (kV) plotted against screen light output (candelas/amp) of a specimen screen, Figure 4 is graph of back-scattered electron energy distribution of two specimen screens, one (broken lines) of a screen backed with aluminium only and the other (full line) of a screen backed with aluminium and carbon, Figure 5 is a graph of contrast measurements made using a carbon/aluminium backed screen, and Figure 6 is a graph of contrast measurements made using an aluminium backed screen.

Figure 1 illustrates diagrammatically a display tube of a type disclosed in British Patent Specification No.

1,434,053 (PHB 32324) details of which are incorporated herein by way of reference. The illustrated display tube comprises an evacuated envelope 10 including a faceplate 12. Within the envelope 10 there is provided an electron gun 14 including a cathode 16 for producing a low energy electron beam 18. A large area channel plate electron multiplier 20 is disposed close to the faceplate 12. The construction of the electron multiplier 20 is disclosed in detail in Specification 1,434,053 and the patent specifications referred to therein and accordingly it will be described briefly only since its construction and operation are not essential to the understanding of the present invention.

The electron multiplier 20 comprises a stack of apertured dynodes insulated from each other and maintained at a substantially constant potential difference between successive dynodes. The apertures in each dynode are barrel shape viewed in longitudinai cross-section and if the material of the dynodes is not secondary emitting then a secondary emitting material can be provided in each aperture. The apertures in successive dynodes are aligned.

Deflection means 22 which may be electrostatic or electromagnetic are provided to scan the electron beam 18 across the input side of the electron multiplier 20. A fluorescent screen 24 is spaced approximately 10 mm from the output side of the electron multiplier 20. As the screen 24 is flat and the faceplate 12 is curved then it is necessary to provide a flat, optically transparent support 26 for the screen 24. If the faceplate 12 is flat then the screen can be provided directly thereon. In operation an accelerating field is provided betwen the output side of the electron multiplier 20 and the screen 24.

Figure 2 shows enlarged and not to scale the last dynode 30 of an electron multiplier, an output electrode 31 and spaced therefrom a flat faceplate 12 on which is provided a layer 32 of at least one type of phosphor, a reflective metallic layer or film 34, for example an aluminium layer, and a layer 36 of a material having a low atomic number and low back-scattering coefficient, for example a layer of carbon or boron.

The phosphor and aluminium layers are laid down using techniques which are conventional in the display tube art. The thickness of the aluminium layer is determined to obtain the desired reflection whilst at the same time provides an impervious layer to the material of the layer 36 to prevent it from passing through the layer 34 and affecting the phosphor layer 32, the aluminium layer is typically of the order of 19 Fg/cm2 (microgrammes/cm2) but may have a thickness up to 104 ag/cm2. The carbon layer 36 is deposited by electron beam evaporation or other suitable method. The thickness of the layer may lie in a typical range of 18 yS/cm2 to 62 Fg/cm2, the actual thickness being selected to give the desired contrast.

In Figure 2 the current multiplied electron beam can be seen leaving the last dynode 30 with energies of up to a few hundred volts and being accelerated towards the screen 32 which is at a few thousand volts relative to the voltage applied to the last dynode 30 and at least some electrons being back-scattered electrons from the screen to points spaced from the point of impingement of the incident, current multiplied, electron beam.

For a case in which a carbon layer is applied on top of an aluminium layer having a thickness of the order of 19 Fg/cm2 an empirical relationship between the screen cut-off potential Vc and the electron acceleration potential VA iS as follows: Vc = 0.38VA + K where K is a constant depending on the contast C. Contrast C in turn is defined as the ratio of light output due to back-scattered electrons to the light output due to primary electrons. The values of K are given below: C K 1% 2.4 2% 1.6 3% 0.4 For other thicknesses of the aluminium layer, the values of K can be calculated using techniques for determining the screen cut-off potentials.

Figure 3 is a graph of screen voltage, Vs, in kV against a screen output in candelas/amp of a specimen screen. As shown the screen produces a light output above a threshold value of screen voltage. This means that any electrons with voltages below the threshold value cannot contribute to the light output of the screen. As the screen voltage, Vs, is increased the light output initially increases non-linearly and then substantially linearly. The screen cut-off potential Vc is determined by the intercept of the extrapolation of the linear part of the graph with the abscissa. From this curve the light output Lp due to the primary electron beam can be calculated in terms of candelas per milliamp (cd/mA) for any chosen beam energy.

Figure 4 shows in broken lines a graph 40 of back-scattered electron energy distribution of a screen backed with 66 Fg/cm2 aluminium and in full lines a similar graph 42 of a screen backed with 19 ,ag/cm2 aluminium and 38 ,a9/cm2 carbon, in each case the primary beam energy is 7 keV. The abscissa is energy in keV and ordinate FE is the number of secondary electrons expressed as a fraction of the primary electrons which is taken as unity. It will be noted that not only does the carbon/aluminium backed screen produce less secondaries for the same beam energy compared to the aluminium backed screen but also the peak in the graph 42 is offset towards the ordinate compared to the peak in the graph 40.

In order to decide on the optimum thickness of the carbon layer for a fixed thickness of aluminium, one first selects the desired light output of the screen and using a graph of the type shown in Figure 3 it is possible to determine the cut-off voltage Vc and the EHT necessary to produce this light output. In choosing the brightness one has to have regard to the usual precautions that too high an EHT will cause undesired X-ray generation and too high a beam current will lead to the spot blowing-up due to space charge effects.

Knowing the desired EHT and Vc then one turns to the graphs shown in Figure 4 and determines by integration the light output due to the secondaries having energies between the primary energy 7 keV and the energy Vc keV at the screen cut-off voltage. There is no point in continuing the integration below Vc keV because the screen will not produce any light output for the lower energy secondaries. By way of illustration, from Figure 3 it is possible to determine that the light output at 6 kV is 20 cd/amp.In Figure 4 for electrons having an energy of 6 keV, the aluminium backed screen, graph 40, gives off secondaries in a proportion of 0.025 (or 2.5%) of the primary electrons and these secondaries produce a light output of 0.025 x 20 = 0.5 cd/A, on the other hand the graph 42 gives a proportion of 0.007 (or 0.7%) for the aluminium/carbon backed screen and the contribution to the light output is 0.007 x 20 = 0.14 cd/A. A similar calculation is done for all energy values between 7.0 keV and Vc and keV and the light output is aggregated.

Mathematically the light output La due to the back-scattered electrons is given by

where FE is the fraction of back-scattered electrons with energy E per milliamp beam current, LE is the light output per milliamp, for a beam of electrons with energy E, and Ep is the energy of the primary electron beam.

The back-scatter contribution to the light output as a fraction of the primary beam light output is given by LB/LP- In Figure 5 the back-scatter contribution, B-SC, i.e. the fraction LB/Lp, expressed as a percentage has been plotted against primary beam light output, PBLO, expressed as candelas/mA, i.e. cd per mA over a range of screen backing thicknesses. All the screens S1 to S6 have an aluminium backing layer 19 yg/cm2, but the screens S2 to S6 also have a carbon backing layer. The screen potentials in kV are indicated by broken lines and the inset tabular summaries give the cut-off voltages Vc for the particular screens.

As an illustration, consider a display device in which the primary electron beam has an energy of 10 keV and the screen (S1 ) has an aluminium backing such that the screen cut-off voltage Vc is 3.5 kV, then, assuming that the back-scattered electrons are returned to the screen without loss of energy, the back-scatter contribution to the total light output will be ~6.4% and the light output due to the primary beam will be 125 cd/mA. If instead the screen has an aluminium/carbon backing so that Vc is now 6.7 kV, that is screen S6, then forthe same light output, the back-scatter contribution is reduced to less than 2% whilst the operating voltage must be increased to substantially 13 kV to obtain the same light output.

The carbon layer lowers the back-scatter contribution in three ways.

a) the total number of back-scattered electrons is reduced, b) the mean energy of the back-scattered electrons is also reduced, and c) the cut-off voltage of the screen is increased.

Within the range of thicknesses of the layers covered by experiments made, these effects are greater the greater the backing thickness. Using Figure 5, the maximum backing thickness can be chosen so that a predetermined contribution due to back-scattered electrons will not be exceeded, for example, if the back-scatter contribution should be less than 1% and the maximum beam voltage is 10 kV then one would use a screen S5 which has a cut-off voltage of 6.1 kV and a light output of 75 cd/mA. The screen backing will be 19 Fg/cm2 aluminium and 46 ,uwg/cm2 carbon.

In order to see if a similar result could be achieved more simply by increasing the thickness of the aluminium backing, thus increasing the cut-off voltage, a similar set of curves for aluminium alone are shown in Figure 6.

Considering the previous numerical example of an aluminium backed screen (S1) having a cut-off voltage of 3.5 kV and a beam energy of 10 keV giving a back-scattering contribution of 6.5%, then increasing the cut-off voltage to 7.0 kV using aluminium alone reduces the back-scatter contribution to 4.25% and the operating voltage for the same light output is 13.5 kV. This compares with a contrast loss of 2% and an operating voltage of 13 kVfor an aluminium/carbon backed screen.

Claims (6)

1. A method of improving the contrast of an image displayed on a vacuum image display device containing a fluorescent screen and requiring an accelerating voltage in the vicinity of the screen, comprising applying a metallic reflecting layer to the fluorescent screen to reflect light produced by the screen and applying a layer of an element of low atomic number and low back-scattering coefficient on the metallic reflecting layer to reduce back-scatter of electrons from the screen, wherein the thickness of the metallic layer is selected to be sufficient to obtain the desired reflection and provide an impervious layer to prevent said element from going through the metallic layer into the screen and the thickness of the layer of said element is selected to give the desired contrast.

2. A method as claimed in Claim 1, wherein the metallic layer is of aluminium and said element comprises carbon, and wherein for an aluminium layer having a thickness substantially of the order of 19 ,ug/cm2, the thickness of the carbon layer is determined in accordance with the relationship: Vc = 0.38VA + K where Vc is the cut-off voltage of the screen, VA the electron acceleration potential and K is a constant depending on the contrast C which is defined as the ratio between the light output due to back-scattered electrons to light output due to primary electrons.

3. A method of improving the contrast of an image displayed on a vacuum display device containing a fluorescent screen and requiring an accelerating voltage in the vicinity of the screen, substantially as hereinbefore described with reference to the accompanying drawings.

4. Avacuum image dispay device comprising an evacuated envelope having a faceplate, a fluorescent screen adjacent to, or spaced from, the faceplate, a metallic layer on the fluorscent screen to reflect light produced by the screen towards the faceplate, and a layer of an element of low atomic number and low back-scattering coefficient on the metallic reflecting layer to reduce back-scatter of electrons from the screen, wherein the thickness of the metallic layer is such as to obtain the desired reflection and to form an impervious layer to prevent said element from going through the metallic layer into the screen and the thickness of the layer formed by the element of low atomic number is determined to give the required contrast performance in accordance with the following relationship: : Vc = 0.38VA + K where Vc is the cut-off voltage of the screen, VA the electron acceleration potential and K is a constant depending on the contrast C which is defined as the ratio between light output due to back-scattered electrons to light output due to primary electrons.

5. A device as claimed in Claim 4, wherein the element of low atomic number is carbon.

6. A vacuum image display device constructed substantially as hereinbefore described with reference to the accompanying drawings.